C++: add provisions for optional RTTI support

C++: add provisions for optional RTTI support

Ref. https://github.com/espressif/esp-idf/issues/1684

This change allows RTTI to be enabled in menuconfig. For full RTTI
support, libstdc++.a in the toolchain should be built without
-fno-rtti, as it is done now.

Generally if libstdc++.a is built with RTTI, applications which do not
use RTTI (and build with -fno-rtti) could still include typeinfo
structures referenced from STL classes’ vtables. This change works
around this, by moving all typeinfo structures from libstdc++.a into
a non-loadable section, placed into a non-existent memory region
starting at address 0. This can be done because when the application
is compiled with -fno-rtti, typeinfo structures are not used at run
time. This way, typeinfo structures do not contribute to the
application binary size.

If the application is build with RTTI support, typeinfo structures are
linked into the application .rodata section as usual.

Note that this commit does not actually enable RTTI support.
The respective Kconfig option is hidden, and will be made visible when
the toolchain is updated.